The invention relates to a filter assembly for collecting used toner in an electrophotographic copying machine, and more particularly, to such filter assembly which collects used toner which is swept off an electrostatic latent image carrying member by means of a brush. In an electrophotographic copying machine which employs powder toner as a developer, it is necessary to clean, by utilizing some convenient means, the surface of a member such as a photosensitive drum which carries an electrostatic latent image thereon, by eliminating any residue of used toner which remain attached thereto, after the transfer of a toner image into a record member such as copy paper or the like. At this end, a conventional copying machine includes a brush which is disposed for rotation while maintaining contact with the surface of the member to scrape or sweep any residual toner off the member, a suction fan for producing an airstream which conveys used toner removed by the brush, and a filter disposed in the path of the airstream for collecting the used toner.
The applicant has previously proposed an improved cleaning unit (see Japanese Laid-Open patent application No. 137,352/1979 or patent application No. 46,187/1978) in which the cross-sectional area of the airstream which conveys the toner is increased in the vicinity of the collecting filter to reduce the speed of the airstream to thereby enhance the toner collecting efficiency of the filter and in which the toner is forced to fall down in front of the filter with a toner receiver disposed upstream of the filter to collect the falling toner therein, thus enhancing the toner collecting efficiency and permitting a facilitated maintenance with a simplified construction.
However, the improved cleaning unit, though it exhibits a number of excellent effects over the prior art, experiences an inconvenience when replacing the toner collecting filter. Specifically, the separate provision of the toner collecting filter and the toner receiver requires a separate discharge of the toner which is collected within the toner receiver when the filter is to be replaced. In addition, toner which is deposited on the front surface of the filter or a pile of toner formed immediately in front of the filter may be left within the body of the copying machine, causing a possible contamination of the surrounding environment. These possibilities may give rise to a fatal damage to the electrophotographic copying machine. Another difficulty with this construction is caused by a positioning frame disposed along the inner periphery of a duct in which the filter is disposed. This may leave a clearance between the duct and the filter to provide a bypass path through which part of the toner may be withdrawn by the fan, thus possibly marring a copy sheet.